1. Technical Field
The present invention generally relates to a device for heating batteries. More particularly, the present invention is directed to an electrical circuit and mechanism using an inverter circuit and the battery main disconnect circuitry to heat the battery or a group of batteries.
2. Discussion
It is well known that most electrochemical batteries including lead acid, NiCd, NiMH, or Li-Ion, Li-Polymer, etc. with potential use in electric, hybrid electric, or in conventional vehicles typically need some form of heating at cold, particularly extreme cold temperatures before they can deliver their full power capability. Traditional methods of warming up starter batteries in the cars have included leaving the headlights on for a few moments or even applying a short at the battery terminals momentarily to warm up the battery. These methods, including those using an external energy source, such as a heated jacket, to warm up the battery, tend to waste a substantial amount of energy outside the battery in order to obtain a proportionately small increase in the battery internal temperature.
Research and investigation has shown that the most efficient way of self-heating a battery is through exchanging energy back and forth between the battery and an external energy storage device such as an inductor or a capacitor, or a combination thereof. An exemplary circuit for performing this energy exchange for battery self heating is disclosed in U.S. application Ser. No. 09/070,331, filed Apr. 30, 1998, which is commonly owned and expressly incorporated herein by reference. An even simpler way of implementing this concept is by passing an alternating current through the battery, which is effectively a constant voltage source.
Laboratory experiments have shown that, depending on the battery chemistry, the internal impedance of the battery at −20° C. (below freezing) drops to as much as half when subjected to a 10 Amp 60 Hz current in less than 50 seconds for a 6-10 Ah battery. This means that the power delivery capability of the battery doubles in less than 50 seconds. Increasing the frequency as well as the magnitude of the current applied can substantially reduce this time, but the impact on the battery life may be adversely affected.
In hybrid electric vehicles (HEV), the most effective way of warming the battery in subfreezing temperatures is through charging. For example, the vehicle's engine (gas, diesel, etc.) will propel the electric motor in the “generator mode” and the motor inverter in the “rectifier” mode to effectively charge the vehicle's battery using the engine's power. Since the impedance of the battery rises considerably in the extreme cold, the charging or flow of current through the battery automatically warms up the battery internally much more effectively than in above-freezing temperatures.
This option is not available in pure electric vehicles (EV) due to the absence of the supplemental engine or power source. In view of this limitation with electric vehicles, it is desirable to provide a circuit which is capable of warming the battery used for powering the electric vehicle during cold environmental conditions. It is also desirable to provide a circuit which can be easily incorporated with the existing power circuitry for the electric vehicle. Finally, it is desirable to provide a circuit which provides a dual function within the electric vehicle for minimizing the number of components within the electric vehicle's power circuit.